Hydrothermal Synthesis of Open-Framework Borophosphates with Tunable Micropore Sizes, Crystal Morphologies, and Thermal Stabilities

Thermal
stabilities of zeolitic frameworks are important parameters
for many applications. Two decades of research have produced only
a very small number of zeolitic borophosphates such as Na2[VO­(B2O)­(PO4)2(HBO3)]·xH2O (x ≈ 2.92) (denoted as B3P2), which shows the onset dehydration and
a complete decomposition at 200 and 400 °C, respectively. In
order to enhance thermal stabilities of borophosphate frameworks,
a water-deficient hydrothermal route with phosphoric acid as the sole
solvent has been developed and led to controlled syntheses of B3P2 and a new vanadium borophosphate, K1.33Na0.67[VO­(B2O)­(PO4)2(HPO4)]·xH2O (x ≈ 1.63) (denoted as B2P3). The latter is the first-ever borophosphate
possessing the zeolite RHO-type net and is characterized by superlarge
spherical cages, including 16-ring and 8-ring channels along the axes
and 12-ring channels along the diagonals of the cubic cell. The new
compound B2P3 has larger structural cages and higher thermal
stability than B3P2, where the enhanced thermal stability is
attributable to different bonding arising from the substitution of
[BO2(OH)] by [PO3(OH)] in the framework. This
is the first demonstration that the micropore size, crystal morphology,
and thermal stability of zeolitic borophosphates can be tuned by changing
the fundamental building units of their frameworks via adjusting the
B/P ratios in the starting materials.